One type of inkjet printer well known in the art forms images on a recording medium by repeatedly and alternately executing a recording operation for reciprocating a print head in a main scanning direction while the print head ejects ink onto the recording medium, and a conveying operation for conveying the recording medium in a subscanning direction. However, this type of inkjet printer does not have a recording head with a nozzle pitch capable of matching recent improvements in recording density. Thus, to improve recording density with this inkjet printer, techniques have been employed to eject additional ink droplets for filling gaps between the nozzles.
Japanese Patent Application Publication No. 2002-283543 discloses one such method for recording with a recording head having N nozzles formed at a nozzle pitch L. In order to achieve a recording density of L/3 in the subscanning direction with this technique, recording is performed while conveying the recording head L/3 in the subscanning direction twice and subsequently conveying the recording head (N−1+⅓)L in a subscanning direction and repeating this operation.
However, error is produced in the conveying distance of the recording medium because the conveying rollers cannot convey the recording medium according to the theoretical value. Specifically, the conveying rollers have mechanical error due to the shape or eccentricity of the roller, Consequently, while the conveying roller rotates one time from a predetermined rotating start position, error in the conveying distance of the recording medium caused by the conveying roller changes within one cycle of a sine wave with the rotating start position set to 0.
The technique disclosed in Japanese Patent Application Publication No, 2002-283543 described above sets one unit of conveyance as a combination of two conveying operations at L/3 and one conveying operation at (N−1+⅓)L. When a plurality of recording operations have been performed in these units of conveyance, recording in one unit of conveyance have deviated from recording in another unit of conveyance. This positional deviation produces what is called “banding” (irregular streaks of ink), preventing this technique from forming high-quality images.